Brake Rotors attach directly to your wheel hub and rotate as one with it. When you press on the brake lever, the brake pads will cause friction on the rotor's surface and your bike will slow down. Brake rotors are specifically designed to prevent heating up on long descents, with many cut outs and wavy patterns in an effort to keep your brakes cool and consistent regardless of how much braking you need to do.
Shimano recently launched "Ice Tech" Technology that uses Steel and Aluminium to disipate heat faster than ever. Shimano is also home to Centrelock rotors, ditching the 6 bolt attachement to the hub, the centrelock makes for a faster assembly and uniform brake pressure distribution on the hub. The Shimano RT81 (Centrelock) and Shimano RT86 (6 Bolt) are massively popular amoungst mountain bikers.